1. Field of the Invention
The present invention relates to the production of printed circuit boards and, more particularly, to a method and device for producing multilayer printed circuit boards, under the application of heat and pressure to a stack of circuit boards and interposed layers of thermosetting plastic.
2. Description of the Prior Art
For a variety of reasons, including the shortening of the conductor length in data processing applications, it has become advantageous and necessary in the electronics industry to utilize multilayer printed circuit boards which consist of a number of separate circuit boards arranged in a permanent stack. Each circuit board carries a pattern of conductors the manufacture of which may involve an etching process, for example. However, this manufacturing process as well as the conductor pattern on the circuit board are not relevant to the present invention.
In order to obtain a permanent bond between the superposed printed circuit boards, it is necessary to arrange between the circuit boards thin plates or sheets of thermosetting plastic material which, under the application of heat and pressure, are brought to a state of flux from which they are allowed to cure and harden to an irreversibly solid state. Once solidified, these layers of plastic material form a strong, permanent bond between the superposed printed circuit boards.
This heating and curing process necessitates the controlled introduction of heat and the coordinated application of pressure to the circuit board stack. Initially, the circuit board stack is heated, under moderate compression, to the point at which the bonding layers enter into a state of flux. After a short period of continued heat application, during which the temperature rises above the flux point, the circuit board stack is subjected to a greatly increased level of pressure. This maximum pressure is then maintained over a predetermined period of time. The simultaneous application of maximum heat and pressure is followed by a cooling cycle, under the continued application of pressure, until the bonding layers between the circuit boards are stabilized and permanently solidified.
Depending on the number of printed circuit boards which are to be assembled in a multilayer package, the prior art teaching in this field suggests the use of one or the other of two different assembly methods. Both methods have distinct advantages, but the advantages are in each case achieved at the cost of certain shortcomings.
For a circuit board stack which has up to three circuit boards and, under favorable conditions, as many as four circuit boards, it is possible to perform the heating and cooling cycles in two separate presses, the first being a heating press and the second being a cooling press. The advantages of this approach are obvious: The heating press does not require a cooling circuit and its temperature can be maintained at a high level. The use of a separate press for the cooling cycle also greatly improves the operating efficiency of the installation by allowing for the simultaneous heating of one multilayer stack and cooling of another multilayer stack.
The major shortcoming of the described method resides in the fact that the use of two separate presses requires the interruption of the compression on the multilayer stack and the transfer of the stack to the cooling press, at the maximum temperature. Following this transfer, the multilayer stack is again compressed and its temperature is lowered to the vicinity of the ambient temperature. Compression during the cooling cycle is necessary, in order to minimize displacement and/or warping of the printed circuit boards in the multilayer stack, due to the temperature gradient which is generated by the cooling process.
A major risk of this assembly method lies in the transfer of the loose and hot multilayer stack from one panel press to the other, in a state in which the bonding layers are still in a state of flux and the stack is consequently very susceptible to shifting of the printing circuit boards. It has been found that this problem is generally manageable with only three circuit board layers, but that this approach is unsuitable for stacks of more than four printed circuit boards.
For multilayer stacks of five and more circuit boards, experience has shown that it is necessary to resort to a different assembly method which involves a heating and cooling treatment of the multilayer stack in the same panel press, without pressure interruption, in order to avoid the risky stack transfer in the pressure-free state. The need for a continuous application of pressure to the multilayer stack, while assuring a distortion-free assembly, has several obvious disadvantages: Firstly, this method is very time-consuming, because the heating and cooling cycles have to be performed successively, in a single extended treatment process. This process is also very energy-intensive, making it necessary for the plates of the panel press to be heated from the ambient temperature to the maximum temperature and then to be cooled all the way down to the vicinity of the ambient temperature, before the finished multilayer stack can be removed from the panel press.
The use of separate heating and cooling presses has obvious advantages in terms of operating efficiency and related cost savings. On the other hand, the risk of producing costly rejects, due to distortions suffered in the pressure-free transfer of a multilayer stack, at or above the flux temperature of its bonding layers, has previously made it mandatory to resort to the more expensive and time-consuming second method, whenever more than four circuit boards had to be assembled into a multilayer stack.